Hydrolysis of interpolymers of ethylene and an organic monocarboxylic acid ester of vinyl alcohol is described in U.S. Pat. No. 2,386,347. The interpolymer is dissolved in an alkaline liquid medium in which it is hydrolyzed.
Solid phase hydrolysis of a copolymer of an olefinic hydrocarbon and an ester of unsaturated alcohol is disclosed in British Pat. No. 1,095,204 and U.S. Pat. No. 3,510,463. In this process, the solid copolymer is heated with a hydrolyzing agent, preferably a lower alkyl alcohol containing an alkaline reacting catalyst such as an alkali metal alkoxide until alcoholysis is achieved. The solid copolymer does not dissolve but is transformed into the hydrolyzed or alcoholized product. This process is attractive because of its simplicity, but, unfortunately, has one very undesirable feature. The product obtained, especially at high degrees of hydrolysis, is discolored yellow, orange, or golden brown. This color is not readily removed by washing or extracting with alcohols or other solvents. It is undesirable that the hydrolyzed resin product be discolored because the articles molded therefrom will also be discolored to a similar extent.
The hydrolysis process is accomplished by an alcoholysis or transesterification reaction in which, for example, an ethylene-vinyl acetate copolymer is converted to the hydrolyzed product and the acetate ester of the alcohol used, e.g. methyl acetate from methyl alcohol. In virtually all cases, the hydrolyzed product is, in fact, a terpolymer composed of repeating units of ethylene, vinyl alcohol, and a small amount of unhydrolyzed vinyl acetate. The problem of yellow to orange to brown discoloration in the hydrolyzed product becomes a serious when the degree of hydrolysis is relatively high, for example, when, in the case of ethylene-vinyl acetate copolymers containing 3.50 weight percent vinyl acetate, the hydrolyzed product contains 0.1-15 weight percent residual vinyl acetate. Usually the color retention is most severe in hydrolyzed products containing 0.1-5% residual vinyl acetate.
The degree of discoloration produced in the solid phase alcoholysis of ethylene-vinyl ester copolymers also becomes more serious as the amount of catalyst used to effect the reaction is increased, or as the volume of the alcoholysis medium is decreased. Generally, for a given alcoholic medium, the lower the vinyl ester content of the copolymer, the greater will be the amount of catalyst required to effect the hydrolysis in a given period of time. Consequently, the lower the vinyl ester content of the ethylene-vinyl ester copolymer hydrolyzed, the greater will be the amount of discoloration in the product.
We have now found that exposure of the material hydrolyzed and/or the hydrolyzing agent to irradiation acts to prevent the development of color. However, it is not apparent why the irradiation suppresses the color build-up in the hydrolyzed resin product. We have found that the hydrolysis medium always becomes discolored yellow to brown as the alcoholysis proceeds whether or not radiation is employed. Evidently, the irradiation does not operate by destroying all color in the medium. Possibly the objectionable color bodies are destroyed selectively or they are modified in some way such that they are not readily absorbed onto the hydrolyzed resinous product.
It is the object of this invention to provide a process for the production of hydrolyzed olefin-vinyl ester interpolymers which are colorless, i.e., having a yellowness index of less than 30. This and other objects of the invention will become apparent to those of ordinary skill in the art from the following detailed description.